1. Field of the Invention
The present invention relates generally to remote control maneuvering of vehicles, and more specifically, to proximity operations between a relatively large target vehicle and a relatively smaller service vehicle. The service vehicle can have a multitude of specific missions, such as re-fueling the target vehicle, providing replacement components to the target vehicle, providing course corrections or other steering functions for the target vehicle, and even simple inspection.
2. Description of the Related Art
In a variety of fields, vehicles capable of sophisticated functionality and/or operating in hostile or remote environments tend to be costly to build and place in use. In no other field is this more dramatic than space vehicles, such as satellites, which can cost hundreds of millions of dollars. Satellites costing hundreds of millions of dollars to produce can also cost hundreds of millions of dollars to launch. Launch costs are directly related to the size or weight of the payload.
Occasionally a satellite can become either completely or partially inoperable due to system wide or component-specific failures. Even in the absence of an unexpected failure, satellites are considered to have finite, and relatively short, lives due to the limited supply of fuel for operating thrusters (which are required to maintain a desired orbit and attitude), and the limited life span of moving parts or heat-generating parts, such as momentum wheels that are used for attitude control. Batteries and solar cells are also known to have only so many charge/discharge cycles before they too become either inoperable or of diminished capacity.
In the past, it has been known to employ human resources, i.e., astronauts, on manned space missions, to dock with a troubled satellite to initiate a repair. Probably the most notable example of that would be the Hubble Space Telescope where an optical correction package was installed by what could be described as a conventional proximity operation.
FIG. 1 illustrates a conventional proximity operation in which a target vehicle 10 is presumed to have some need for a proximity operation. In the case of the Hubble Space Telescope, the target vehicle would be the Hubble satellite which, schematically illustrated, would include a sensor component 12. The sensor component 12 for the Hubble Space Telescope is an optical telescope that includes a plurality of optical elements.
When it became clear that a repair to the optical elements was required of the Hubble Space Telescope, NASA launched the Space Shuttle as a service vehicle 14 with the mission of repairing the sensor 12. In that case, the service vehicle 14 was manned, relatively large, and relatively expensive to launch and provision. The service vehicle 14 included an on-board sensor 16 for sensing position and velocity of the target satellite 10 for the purpose of making proximity maneuvers. The service vehicle 14 also included a processor module 18 for taking the position and velocity data of the sensor 16 and converting that data into command signals for thrusters, such as thrusters 20, 22, and 24, which can be differentially operated to provide a desired approach for docking. The service vehicle 14 also includes an attitude control system 25 which can also be used to precisely position the service satellite during docking or for precise inspection vectors.
A cooperating docking means allows the two satellites to be coupled together during the repair operation. A typical docking means could include a post 26 which is grabbed by a grappling arm 28.
Thus, the repair mission performed in the past, as demonstrated by the Hubble Space Telescope mission, entails the following operations. First, the service vehicle is launched. Following launch, the service vehicle will rendezvous with the target vehicle. During the rendezvous, the two vehicles come into proximity to each other as a result of orbital software. The orbital software takes into account the dynamics of the two orbits, meaning the orbit of the target vehicle and the orbit of the service vehicle. Orbital software allows the service vehicle to undergo a series of maneuvers to get somewhat close to the target vehicle; depending on the size of the vehicles, “close ” can be anywhere from 1 to 100 kilometers, although greater or lesser distances can be foreseen.
Once rendezvous has been completed, for a manned mission, a pilot or astronaut takes manual control of the service vehicle and completes maneuvers either to perform the inspection sequence, or to bring the service vehicle into close proximity, meaning, close enough for the software and control to take over. Thus, for example, at the end of the proximity maneuvers, the two vehicles are ready to dock. Once docking has been completed, the repair is made by manual replacement of the sensor 12 on the target vehicle 10.
A number of patents describe maintenance and/or repair operations in space. For example, U.S. Pat. No. 5,421,540 to Ting describes a method and apparatus for recovering space debris, in which recovery vehicles are launched, and maneuvered to come into close proximity with a target object, which may-be a satellite or simply debris. As described therein, the sensing and maneuvering aspects of the proximity operation are carried by the repair or recovery vehicle. Such arrangements tend to require large, complicated vehicles that are relatively expensive to build and launch.
U.S. Pat. No. 4,298,178 to Hujsak describes a roving geosynchrounous orbit satellite maintenance system, in which a maintenance vehicle docks with a larger vehicle and off-loads replacement equipment. The vehicle includes a rotatable cradle containing part modules which can be rotated into position so that parts can be off-loaded.
U.S. Pat. No. 4,079,904 to Groskopfs et al. describes in greater detail some known docking structures that might be used in vehicles such as the Space Shuttle. U.S. Pat. No. 6,364,252 to Anderman describes a technique for implementing a particular rendezvous sequence or operation.
While the aforementioned proximity operations, particularly the Hubble repair mission, can be very effective at making certain repairs, they can also be so costly as to bring into question whether it might be less expensive to launch a replacement of the satellite 10, rather than make repairs to it.